Tuner I/Fs

Subject: TV Tuners, RF and I/F debugging. When the I/F goes bad, or when you fail to program a tuner properly.

United States Digital CableTV signals generally look like this first image. Actually, this is how almost every digital TV signal looks around the world, but I digress. They have a frequency and a bandwidth. Frequency in the image is 579MHz and the Bandwidth is 6MHz.

TV Tuner components take a slice of RF signal at a user supplied frequency F(MHz), of bandwidth B(MHz) and move that signal to an ‘Intermedia Frequency’, much lower in the RF spectrum, typically around 5-6MHz, or higher at 44MHz. Never any higher than 44MHz.

The tuner also isolates the signal by removing anything that falls outside of F ± B/2, that’s important. So if F is 579MHz and B is 6MHz, and you tuned to a busy cable plant with lots of adjacent signals/channels below 576MHz or above 582MHz, then these adjacent signals would be discarded by the tuner. This is generally called channel rejection.

The tuner outputs a new Intermediate Frequency (I/F) at 6MHz, just like this second image. In this case we took a probe from a spectrum analyzer and measured the I/F out of the tuner to demonstrate the signal has moved from 579MHz to 6MHz.

Due to the fact TV Tuners sample and down-convert signals to I/F’s, and the I/F is passed from the tuner to the Digital Demodulator, the job of the demodulator is greatly simplified. I won’t discuss demodulators here, as I want to focus on "When things go wrong with tuners!".

Look at this third image, its a measurement taken from a tuner project I recently worked on. For the life of me I could not understand why my demodulator was not locking. I scratched my head for a few minutes then tried to figure out whether I had a tuner programming issue, resulting in a poor I/F being produced by the tuner (and given to the demodulator) or a demodulator programming issue. The answer, ALWAYS, is to inspect the I/F coming from the tuner.

Images two and two should be generally identical, clearly something was wrong. Clearly the I/F coming from the tuner looked bad, it looked nothing like a digital television image. I isolated the problem to the tuner, not the demodulator and fixed the tuner programming issue within a few minutes. Had I not done that, I could have been banging away - trying various demodulator fixes, with no effect.

Lessions learned, if you’re have locking issues, check your tuner I/F output when you can. Its a time saver.